The ssrA gene, which encodes a small stable high copy number RNA transcript (tmRNA), is found in all bacteria and has recently been identified in chloroplasts and diatoms. It has a dual function both as a tRNA and as an mRNA molecule and is involved in rescuing truncated mRNAs which have lost stop codons, facilitating trans-translation of truncated peptides prior to protease degradation (Keiler, K. C. et al. (1996), Science, 271, 990-993). The unique function of tmRNAs has directed researchers to analyse the relationship of the secondary structure of these molecules with their function. These studies have focussed on the conservation of the secondary structure of tmRNAs from different microorganisms, and on the evolutionary significance and functional_relevance of such structural conservation. Studies were carried out by Matveeva, O et al (1998), Vol. 16, No. 13, 1374-1375 to investigate oligonucleotide binding to RNA molecules using tmRNA as a model of RNA containing secondary structure. The studies did not have as their objective the identification of sites in tmRNA with the goal of designing antisense oligonucleotide for therapeutic purposes.
The number of nucleic acid targets/probes for bacterial diagnostics is currently limited. As such, the need to identify and characterise novel DNA and RNA targets for diagnostic purposes is now seen as a priority. Target nucleic acid sequences for the development of probes can be for example, plasmids, ribosomal RNA genes, intergenic regions, genes encoding virulence factors or random genomic DNA fragments. In addition, a number of RNA molecules have been described which are used as targets for RNA-based detection for example, ribosomal RNA and RNase P.
The basis of any nucleic acid-based probe assay is the requirement for well characterised nucleic acid sequences which are present in all prokaryotes and eukaryotes under study. For reliable detection of a prokaryotic or eukaryotic organism, the nucleic acid probes used should be highly specific (i.e. not cross-react with nucleic acids from other organisms) and highly sensitive (i.e. most or all strains of the organism to be detected should react with the probe). Therefore, preferred target sequences would be present in all strains of the organism concerned. Such sequences would have significant sequence variability to allow differentiation of the species concerned from other closely related species but, on the other hand, have sufficient sequence conservation to allow the detection of all strains of the species concerned. In general, the precise identification of a nucleic acid sequence, which could form the basis of a specific nucleic acid probe assay, is tedious, difficult and uncertain. To date there are few general approaches which would facilitate the development of nucleic acid probes for a wide variety of microorganisms. The nucleic acid sequences which have been identified as potentially useful targets for probe development are, for example, rRNA genes and RNA, and the rRNA 16S/23S intergenic region.
The majority of nucleic acid probe/target assays centre on the high copy number ribosomal RNAs (rRNA) and rRNA 16S/23 S spacer regions (European Patent No. 0 395 292) of the bacterial cell for the purposes of detection and identification. A number of successful commercial bacterial diagnostic kits have been marketed based on these rRNA probes/targets for the detection of a variety of microrganisms. These include a range of commercial probe kits based on the 16S rRNA gene marketed by Gen-probe Inc. San Diego Calif., and DNA probes based on the 16S/23S spacer region marketed by Innogenetics N.V. Ghent, Belgium. However, many of these diagnostic kits have limitations, including lack of sensitivity due to low copy-number target sequences and lack of specificity due to sequence identity between closely related organisms in many cases.
Nucleic acid-based methods that could be applied directly to samples to give an indication of the viability of any microbes present therein would be of enormous significance for food, industrial, environmental and medical applications.
A disadvantage of DNA-based methods is that they do not distinguish between living and dead organisms. Some studies have focussed on using rRNA and mRNA as indicators of cell viability (Sheridan, G. E. C. et al. (1998) Applied and Environmental Microbiology, 64, 1313-1318). However, these sequences are not satisfactory targets as rRNA and mRNA can be present in bacterial cells up to 48 hours after cell death.
With the advent of nucleic acid based microarray-like formatting, incorporating simultaneous monitoring of multiple nucleic acid targets, there is now a clear requirement to identify and characterise novel nucleic acid sequences for use as probes and/or target regions to detect and identify viable prokaryotic and eukaryotic cells.